It is well known to use screws and nails, or similar pin-type fasteners, for securing floor, wall and ceiling panels to supporting structures in buildings and vehicle cargo container bodies. In the case of truck trailer and cargo container bodies, hard wood floors are attached to a steel metal frame or substrate with screw fasteners. It is essential that those screws be driven in far enough to secure the flooring member in tight engagement with the underlying frame, and also that the heads of the screw fasteners not protrude above the wood flooring. One technology for securing floors to truck trailer bodies requires pre-drilling holes in both the wood flooring and the underlying metal frame members, e.g., steel angle irons, I-beams, U-channels or flat metal bars, and applying self-tapping screws through those holes to anchor the flooring to the frame. That procedure using predrilled holes is slow due to the need to pre-drill the members to be secured together. Accordingly attention has been directed to avoiding the necessity of pre-drilling the underlying frame members.
One prior approach to securing hard wood flooring or panels to aluminum or mild steel substrates or structural frame members without pre-drilling has been to use an electrically powered screwdriver operating at a relatively high speed, e.g., about 2400 rpm, and special high carbon steel self-drilling, self-tapping wing screws, e.g. the fastening system of Muro Corporation of Tokyo, Japan comprising its model FLVL41 electrically powered screwdriver and its Super Wing™ self-drilling screws. Such prior fastening systems are suitable for driving the self-drilling, self tapping wing screws through wood into aluminum frame members; and also suitable for driving screws through wood into mild steel frame members in thicknesses up to about ¼ inch, although that driving operation is relatively slow, typically taking about 15 seconds or more.
However, freight trailer body manufacturers now prefer to use frame members made of a high tensile strength steel characterized by a tensile strength of 80,000 psi and a yield strength of approximately 50,000-65,000 psi (hereinafter “HTS steel”), with the frame members having a thickness of at least ⅛ inch and as much as ⅜ inch. Prior electric powered screw driving systems using self-drilling wing screws have not been satisfactory for reliably, effectively and rapidly penetrating such high tensile strength frame members with self-drilling, self-tapping screws, and there use in such applications has been limited to screws with a maximum shank diameter of about 6 mm, exclusive of the wings, and a maximum head diameter of about 11 mm. In addition to the screws not readily penetrating the HTS steel and the screw driving process being slow (about 25 seconds), a significant limiting factor is that driving the fasteners requires the operator to exert a substantial downward force on the driver in order for the screw driver bit to drive the fastener into the high tensile strength frame member, which leads to operator fatigue. In this connection it is to be noted that the time required to drive each screw is a function of the operator-generated force exerted on the screw by the driver, the torque applied to the screw by the electrically-powered driver, and the rotational speed imparted to the screw by the driver. It is to be noted that also using an electric screw driver with a greater speed, e.g., 5000 rpm, has been found to be unacceptable since at that higher speed the heat buildup at the interface of the screws with the HTS steel frame member will cause the tips of the screws to burn up, which in turn reduces the ability of the drill portion of the screws to penetrate the steel frame member and consequently extends the time required to properly anchor the screw in that steel member. That same tip burning phenomenon has been found to occur to some extent even with drilling speeds of about 2400 rpm.
One effort to overcome the foregoing limitations of prior electric power screwdriver systems with respect to anchoring hard wood members to high tensile strength frame members is the apparatus described in my U.S. Pat. No. 6,990,731, which is incorporated herein by reference. That apparatus, which was found to be capable of rapidly driving high carbon steel self-drilling, self tapping wing screws through hard wood flooring members into high tensile strength steel frame members, comprises a pneumatically-powered impact screw driver carried by a pneumatically operated telescoping driver support/fastener positioning unit for urging the screw driver device to move in a direction that applies an axial driving force to screws. The telescoping apparatus reduces the effort that the operator needs to exert to urge the driver into engagement with the workpiece. Although that system can complete a fastening operation through ¾ to 1⅜ inch thick hardwood flooring into ⅛ inch thick HTS steel frame members using screws with a maximum shank o.d. of about 6 mm in about 12 seconds without any pre-drilling and with much less muscular effort than required with the Muro system, it has several limitations. For one thing it requires a relatively large air flow at a relatively large pressure to assure adequate stall torque and rotational speed for the driver, e.g., about 30 cubic feet/minute air flow at a pressure of about 90 psi. That requires the apparatus to be coupled to a relatively large compressor, e.g., a 25 HP electrically powered compressor. Another limitation is that the high speed of the pneumatic impact driver, about 5000 rpm when supplied with air at a pressure of about 90 psi, tends to cause burning up of the tips of the screws, particularly when attempting to install screws with a diameter in excess of 6 mm, e.g., screws with a shank diameter, exclusive of wings, of 8 mm (which are preferred for trailer truck body applications due to their greater holding power and strength). That tip burning problem can be alleviated by reducing the air pressure to decrease the driver's torque and rotational speed, but at a cost of an increase in the time required to fully drive a screw. Other limitations of that apparatus are the cost of pneumatic impact drivers and the noise that they generate.
Accordingly there has still existed the need for a screw driving and fastening apparatus that is capable of rapidly and reliably driving self-drilling, self-tapping screws into high tensile strength steel substrates having a thickness in the order of ⅛ inch to ⅜ inch without any need for pre-drilling the HTS steel substrates.